A conventional hard disk drive (HDD) can include a system clock and an internal clock. Typically, an internal oscillator can be used as a clock source for the internal clock. Due to variations in manufacturing or processing of the resistive and capacitive elements used in internal oscillators, oscillation frequencies can vary for different internal oscillators. Therefore, the internal clocks in HDDs may not be an accurate source of time.
As the size of conventional HDDs decrease, requirements for emergency parking of read/write heads of the HDDs can become more strict. During emergency parking, a motor controller can use a pre-determined sequence (e.g., a sequence determined from timing and voltage information) to safely land the read/write heads on non-data landing zones after the HDD has stopped operating. Typically, the system clock is used for timing. However, the system clock should not be used if the system clock is, for example, irregular, missing, or not powered.